Feed stock for naphtha reforming



Nov. 27, 1956 J. D. BUSHNELL ET AL FEED STOCK FOR NAPHTHA REFORMINGFiled Sept. 25, 1952 HOT VAPORS FRESH OR C FEED 10 ST l2AG Fr Lit-2T:K20

1 1::::'::; 6 f PREHEATER VAPORIZER INHIBITOR a ames D.-Bush.ne11 PobertL. B qnventors '2 B5 attor neg United States Patent FEED STOCK FORNAPHTHA REFORMING James D. Bushnell, Elizabeth, and Robert L. Berg,Westfield, N. 1., assignors to Esso Research and Engineering Company, acorporation of Delaware Application September 25, 1952, Serial No.311,342

5 Claims. (Cl. 196-50) This invention relates to a process for the heattreatment of hydrocarbons. More particularly, it relates to a processfor the thermal or catalytic reforming of a hydrocarbon feed stockconsisting essentially of saturated hydrocarbons, i. e., onesubstantially free of unsaturates.

The usual thermal or catalytic processes for the con version ofhydrocarbon feed stocks into cracked or reformed products of higheroctane number all require the supply of large amounts of heat. Heat isrequired to raise the temperature of the feed stock to the catalytic orthermal conversion range which includes broadly the limits of about 800F. to 1150" F. for the typical process of this type. In many casesadditional heat is required to vaporize the feed stock as a part of thestep of raising it to reaction temperature. Furthermore, in most ofthese catalytic or thermal processes dehydrogenation or crackingreactions play a predominant role so that the process is more or lessstrongly endothermic. Thus a still further amount of heat must besupplied to furnish the necessary heat of reaction.

In laboratory test runs, and even in some small-scale plant productionit is possible to heat the reaction zone externally so that a coldliquid oil can be fed and the necessary heat for vaporization,preheating and reaction all supplied in this way. In present-daycommercial practice this is not the preferred method, however, for manyreasons. Thus, better results are usually obtained by separating thepreheat zone and reaction zone, so that the reaction can take placeunder more nearly uniform conditions. The recovery of heat from variousprocess streams as preheat to the feed is a particularly importanteconomic advantage, which is to be exploited to the full.

In both the thermal and catalytic types of hydrocarbon conversion,better results are frequently obtained by carrying out the preheatingand vaporization steps in such a way that the feed stock is broughtrapidly up to the final reaction temperature once it exceeds the limitof about 600-700 F. This is the temperature level above which thermaldecomposition begins to take place on longer or shorter exposure,depending of course upon the character of the liquid fed. The amount ofpreheat which can be usefully recovered from a process stream byindirect heat exchange with. the liquid feed is, ordinarily below thisrange. In perhaps more cases than otherwise this limit is not reached,due to a tendency of either the feed stream or the available processstreams to form a deposit on the heat exchangesurface.

In the usual case where there is some danger of such deposits beingformed from unstable or reactive compounds on the feed side of a preheatexchanger, the preheating and vaporization will becarried out in stages,with the first stage limited to a temperature where the feed will remainliquid. Following this initial preheat, the next step is then to passthe preheated feed stock through a second heating section which may be afired furnace or may-involve heat exchange with some higher temperatureprocess stream such as -a regeneration gas,

2,772,220 Patented Nov. 27, 1956 flue gas, or the like. In this heatingzone the naphtha feed stock is vaporized. This means, of course, thatany non-Volatile material present in the naphtha will be deposited onthe surface of the heating tubes or other heat exchange surfaceemployed. For this reason it has been well known in petroleum refiningthat a feed stock to be completely vaporized must be a clean stock" iffrequent shutdowns for tube cleaning are to be avoided.

Although light distillates in the naphtha boiling range are all of themmaterials which have essentially no nonvolatile constituents, petroleumrefiners have also appreciated the fact that feed stocks containingolefinic or diolefinic constituents frequently cause trouble in suchservice. The well-recognized difficnlty here is the tendency of suchunsaturated materials to form peroxides with any dissolved oxygen, andto polymerize with themselves or other reactive constituents of theblend in a gumand resin-forming reaction which is itselfperoxide-catalyzed.

Accordingly, it has been common practice to give olefincontaining feedstocks special handling to prevent gumforming reactions before such acomplete vaporization step. The addition of oxidation inhibitors orblanket ing the processing equipment and storage vessels with an inertgas serves to prevent gum formation, which can be readily detected bystandard methods of test. A naphtha taken from intermediate storage maysimply be steam stripped to remove dissolved oxygen beforefurtherprocessing, particularly when the storage period is short andlittle or no gum has actually been formed. In special cases if it is notpractical to avoid the formation of such gums and resins, the olefinicnaphtha may even be rerun over liquid bottoms to give a clean vaporbefore it is superheated. Such methods of treating these unsaturatedstocks have made it possible in many cases to use an olefinic feed atmoderate temperature levels and still approach the clean exchangerconditions found when using a fully saturated or virgin naphtha feedstock.

The cleanliness characteristic of a virgin naphtha feed has ordinarilybeen taken as the standard in such comparisons, The paraffin, naphtheneand aromatic constituents of such a feed stock are nothing like asreactive with oxygen as the olefins and diolefins. 'They do not formgums and resins on even prolonged exposure to dissolved oxygen, inordinary storage. In the conventional type of thermal reformingoperation where the feed stock has been preheated to a temperature ofthe order of 300 to 350 F. and then introduced into a heated coil forthe reforming reaction, the preheat tubes have remained substantiallyclean over long periods of time, and the useful onstreani period of thereforming process has been limited by coke-forming reactions in thereformer coils.

It has now been found, however, that this same situation does not applywhen substantially higher preheat temperatures are employed. In severethermal reforming, where temperatures well above 1,000 F. are to beemployed, the preheat temperature may go as high as 400500 F., althoughnot ordinarily higher than this In catalytic reforming processes at highconversion levels, however, such as hydroforming or aromatization thereaction is more strongly endothermic and correspondingly more heat mustbe supplied. High level heat is available from the product stream, andthe highest degree of preheat which can be recovered, such as 600650 F.,becomes economically attractive. V

In either type of reforming process, when preheat temperatures of theorder of 400650 F. or higher are employed, it appears that dissolvedoxygen in a saturated naphtha feed stock may become sufficientlyreactive to cause the formation of appreciable amounts of gummydeposits, at elevated temperatures, on the heat exchange 3 surfaces inthe preheat train. It is particularly surprising to discover that thishappens even with feed stocks which are essentially gum-free by standardmethods of test, and which contain no constituents commonly regarded asgum formers. I v

It is an object of the present invention to improve the operation ofaheating unit for preheating and vaporizing a virgin naphtha feed stock.

It is another object of this invention to provide a simple means forexcluding oxygen from a fully saturated fced stock, free of gum-formingconstituents, where this feed stock is to be completely vaporized duringpreheating for a further processing step at an elevated temperature.

It is a further object to provide a means for excluding oxygen from asaturated hydrocarbon feed stock by the addition of an oxidationinhibitor thereto, which accomplishes this aim without requiring the useof special utilities or processing equipment.

It is a still further object to significantly increase the useful timeonstream of a heat exchange unit for preheating and vaporizing asaturated hydrocarbon feed stock in the naphtha boiling range, by addingan oxidation inhibitor to the feed stock in intermediate storage.

According to the present invention a freshly distilled naphtha feedstock, which is being accumulated in storage prior to a subsequentthermal or catalytic reforming process, is treated with from about 0.1to 1.0 lb. or as much as 5 lbs/5,000 gals. of an oxidation inhibitor.The inhibitor should be added preferably to the freshly distillednaphtha, either before or at the same time as the naphtha feed isintroduced into intermediate storage. In this way the possibility ofinteraction between dissolved oxygen and hydrocarbons during storage isminimized, and the feed stream withdrawn from storage for furtherprocessing is essentially free of free oxygen and peroxides. The amountof oxidation inhibitor to be used is substantially the same or somewhatless than that commonly employed for inhibiting an olefinic gasolinewith the same inhibitor.

While various gasoline inhibitors may be used according to thisinvention, the inhibitor should preferably be one having relativelylittle tendency to form gummy deposits itself on vaporization of thefeed containing it. The higher alkyl phenols of the hindered phenol typehave been found to be satisfactory, such, for example, as2,6-di-tertiary butyll-methyl phenol, or the analogous bis-phenols ofthe alklated diphenylol propane type. A fraction of petroleum phenolswhich may be used similarly can be obtained by extracting a heavycracked distillate boiling within the range of from about 250 to 700 F.,with a 30 to 50 Baum caustic soda solution, and acidifying the resultingextract to release the phenols. The N-alkyl phenylene diamines or alkylamino types of inhibitor may also be used. All of these phenol typeinhibitors belong to the general class of poly-substituted aromatics,including the amine-substituted analogs of the phenols. In any case, thechoice of inhibitor will depend upon its tendency to form deposits inthe vaporization step, which depends in turn upon the length of time,temperature and degree of exposure to air in storage, as well as on thefeed stock and the time at temperature in the preheat equipment. v

The invention may be illustrated by referring to the attached drawing,which presents in simplified form a diagrammatic showing of thepertinent parts of a reforming plant. This drawing is constructed sothat it may apply either to thermal reforming or to a catalyticreforming process. The operation will be described first with respect tothermal reforming. g

The fresh feed to the process, introduced through line 10, is ordinarilya freshly distilled,virginnaphtha of low octane quality. The rateatwhich this naphtha feed is produced will not necessarily be the sameas the rate at which it is to be thermally reformed. Whether this is thecase or not a certain amount of the naphtha feed is ordinarilyaccumulated ahead of the reforming step, so as to insure continuousoperation. This accumulation takes place in an intermediate storage tank12, which may run at a constant level or a variable level depending uponhow well the rate of feed production can be synchronized with thereforming process.

According to the present invention, the fresh feed entering this vesselis combined with the required amount or" a gasoline inhibitor, asindicated above, which is equiviii-221i to a concentrationof the orderof 0.001 weight percent. The inhibitor added through line 14 may beintroduced in the feed line before entering the tank or mixed directlyin the feed pump feeding into the tank, or otherwise, so as to insureuniform distribution throughout the material in storage.

Naphtha withdrawn from this intermediate storage tank 12 passes throughpreheater 16, Where it is warmed by heat exchange with any suitableprocess stream. A typical hot process stream for this use might be apumparound reflux stream from a product fractionating tower to beexternally cooled. While a number or hot liquid or gaseous streams maybe available, the use of a liquid stream minimizes the danger of rapidfouling of the outside of the heat exchange surface due to the formationof deposits thereon. Conversely, in many cases, the product vapor streamfrom a high temperature thermal conversion cannot be used this way, inspite of its high heat content, because of a strong tendency to formhigh boiling olefin polymers which may foul the exchanger surface onpartial cooling and degrade the liquid product.

The feed being preheated in exchanger 16 may remain entirely liquid, orit may be partially vaporized, leaving at a temperature of about 450 F.The preheated naphtha is then introduced into furnace 18, where it isfirst completely vaporized and then heated further to reformingtemperature, which may be about 1100 F. In thermal reforming thevaporization section and the heatsoaking section Where most of theconversion takes place will ordinarily be located within the samefurnace 18, as different tube sections. The hot product vapor stream 20leaving the furnace 18 is ordinarily then cooled rapidly to preventfur'th'er undesirable cracking 'or polymerization operations, whichmight result in the formation of additional gas and polymer from productconstituents in the naphtha boiling range.

It will be understood that this description is entirely diagrammatic,and that the usual pieces of auxiliary equipment such as valves, pumps,control devices and product recovery means will be supplied in theconventional manner known to the refining art.

The improvement with which this invention is concerned is in theoperation of preheater 16. The trouble ordinarily encountered here isnot so much actual plugging of the exchanger as it is poor heatexchange. A much smaller amount of a gummy deposit may be enough tocause serious damage in this respect than that which Would result inhighpressure drop, or decreased mechanical through-put. The formation of agummy deposit on these exchanger surfaces takes place largely at andjust after the pointwhere the feed passes its dry point on vaporization.This shortens the run: the preheat outlet temperature goes down and itis necessary to reduce the fee d 'rate through the exchanger in order toget the outlet temperature up to the desired level. The situationfinally reaches the point where the useful feed rate is too low, and itis necessary to shut down to clean out the exchanger surfaces.

It is true that this sort ,oftrouble can be minimized by running to alower preheat temperature so that the deposit in thepreheating train isall formed in the furnace 18. This, however, means usingexpensivefurnace heat instead of waste heat otherwise available. It also adds tothe amount of coke formed in the furnace, which limits the run lengthfinally in any case. The deposit in the furnace ends up as coke, whetherit starts as gum or not, and builds up toward a mechanically limitingpressure drop. At the same time there is a decrease in the usefulthrough-put which is limited by heat transfer through the coked furnacetubes, so that it becomes impossible to get the outlet temperaturesdesired without decreasing the feed rate to progressively lower andlower values. Neverthless, it is much easier to remove coke from thefurnace tubes than it is to clean gum from the preheater surfaces. Thishas resulted in the choice of lower preheat temperatures, even inthermal reforming, than might otherwise be usefully employed.

The process of the present invention is useful in thermal reformingunder a variety of conditions, which may inelude coil outlettemperatures from 800 up to about l,000 to 1150 F. and pressures in therange from about 250 to 1,000 p. s. i. g. The preheat for suchoperations is normally not over about 450 to 500 F., and the process maybe useful broadly where preheat temperatures go substantially above 300to 350 F.

The process may also be used in substantially the same way in catalyticreforming processes, where its use will be described with particularreference to catalytic hydroforming. This operation may be carried outat a reactor outlet temperature of about 900 to 1100 F., operatinganywhere from about 50 to 1,000 p. s. i. g. An important difference inthis case is that the reactor product is fully saturated and need not bequenched, so that the heat content of the product stream can berecovered at a high temperature level in heat exchange with the feed. Atthe same time a very high heat input to the conversion zone is required,to supply heat for the endothermic reaction. For this reason, the outlettemperature from preheater v16 will frequently be as high as 650 to 700F, so that the feed stock is completely vaporized at this stage of theheating process. The preheated vapors are then superheated in furnace 18to a temperature which is ordinarily somewhat higher than the reactortemperature range, and are fed through line 20 into the catalyticrccation chamber.

The fact that the feed stock is completely vaporized in the preheatingexchanger 16 means that the present invention is particularly importantin catalytic reforming processes.

A most significant feature of this invention is the fact that itrequires no special processing equipment. It enjoys an importantadvantage in this respect over previous systems such as steam stripping,rerunning, or inert blanketing, all of which require special utilitiesor separate processing steps for their practice.

It will be understood that while this invention has been described withreference to thermal or catalytic reforming proceses, it may be usefulin a variety of other cases where a saturated feed stock such as avirgin naphtha is to be completely vaporized as part of a process forthe conversion or treatment of hydrocarbons. Such processes might beinvolved in a variety of distillation or solvent extraction proceduresas well as in those involving thermal or catalytic conversions.

What is claimed is:

1. The process of preparing a superheated naphtha vapor feed consistingessentially of saturated hydrocarbons not susceptible to gum formationbelow temperatures of about 400 F. and substantially free of olefinicconstituents for a hydrocarbon conversion process which comprisesaccumulating in storage a body of said naphtha, adding to said naphthaentering storage an amount of gasoline oxidation inhibitor equivalent tofrom about 0.1 to 1.0 pound per 5,000 gallons of liquid naphtha,withdrawing a naphtha stream from said storage, preheating said naphthastream by indirect heat exchange to a temperature above about 350 F. atwhich at least partial vaporization takes place with minimum fouling ofexchanger surfaces, subsequently heating said stream further in aseparate heater to produce a superheated completely vaporized naphthastream, and feeding said naphtha vapor to said hydrocarbon conversionprocess.

2. The process according to claim 1 in which said inhibitor is of thealkylated phenol type. r

3. In a process for converting a virgin naphtha distillate consistingessentially of saturated hydrocarbons not susceptible to gum formationbelow temperatures of about 400 F. and substantially free of olefinicconstituents into a thermally reformed product of improved octane ratingby vaporizing the naphtha and subjecting the vapors to thermal treatmentat a temperature of about 800-1100 F. and at a pressure from about to1000 p. s. i. g., the improved method of preventing fouling of heatexchanger tube surfaces in the vaporization and feed preheat portion ofthe system which comprises adding from about 0.1 to 5 pounds of anoxidation inhibitor per 5,000 gallons of liquid feed to the virginnaphthe in storage prior to said vaporization.

4. The process of preparing a feed stock for catalytic naphtha reformingwhich comprises introducing a freshly distilled virgin naphtha feedconsisting essentially of saturated hydrocarbons not susceptible to gumformation below temperatures of about 400 F. and substantially free ofolefinic constituents for said process into an intermediate storage tanktogether with from about 0.1 to 1.0 pound per 5,000 gallons of naphthaoxidation inhibitor, withdrawing a naphtha stream from said storage,preheating said naphtha stream first by indirect heat exchange to atemperature above about 450 F. at which substantially completevaporization takes place with minimum fouling of the exchanger surfaces,passing said vaporized naphtha through a separate heating zone inindirect heat exchange with hot combustion gases and therebysuperheating the naphtha vapors to a temperature above about 800 F.suitable for said catalytic reforming process, passing said superheatedvapors into a catalytic reforming zone and recovering therefrom a hotproduct vapor stream substantially free of 'olefinic hydrocarbons, andpassing said hot product stream in indirect heat exchange with saidnaphtha withdrawn from storage for said first-named heatexchanging step.

5. The process according to claim 4 in which said inhibitor is of thepolysubstituted alkylphenol type.

References Cited in the file of this patent UNITED STATES PATENTS1,956,567 Ellis May 1, 1934 2,116,773 Voorhees May 10, 1938 2,349,473Tannich May 23, 1944 2,472,463 Brandon June 7, 1949 2,638,412 BrowderMay 12, 1953

1. THE PROCESS OF PREPARING A SUPERHEATED NAPHTHA VAPOR FEED CONSISTINGESSENTIALLY OF SATURATED HYDROCARBONS NOT SUSCEPTIBLE TO GUM FORMATIONBELOW TEMPERATURES OF ABOUT 400* F. AND SUBSTANTIALLY FREE OF OLEFINICCONSTITUENTS FOR A HYDROCARBON CONVERSION PROCESS WHICH COMPRISESACCUMULATING IN STORAGE A BODY OF SAID NAPHTHA, ADDING TO SAID NAPHTHAENTERING STORAGE AN AMOUNT OF GASOLINE OXIDATION INHIBITOR EQUIVALENT TOFROM ABOUT 0.1 TO 1.0 POUND PER 5,000 GALLONS OF LIQUID NAPHTHA,WITHDRAWING A NAPHTHA STREAM FROM SAID STORAGE, PREHEATING SAID NAPHTHASTREAM BY INDIRECT HEAT EXCHANGE TO A TEMPERATURE ABOVE ABOUT 350* F. ATWHICH AT LEAST PARTIAL VAPORIZATION TAKES PLACE WITH MINIMUM FOULING OFEXCHANGER SURFACES, SUBSEQUENTLY HEATING SAID STREAM FURTHER IN ASEPARATE HEATER TO PRODUCE A SUPERHEATED COMPLETELY VAPORIZED NAPHTHASTREAM, AND FEEDING SAID NAPHTHA VAPOR TO SAID HYDROCARBON CONVERSIONPROCESS.